LTSpice simulator of my oscillator is far from reality; bad models?

On Fri, 04 Apr 2014 14:03:58 +1100, Clifford Heath
<no.spam@please.net> wrote:

[snip]

You apparently have multiple line-wrap issues in your .asc file.

Try not putting everything in one line ;-)
		
                                        ...Jim Thompson
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On Fri, 04 Apr 2014 13:58:46 -0700, Clifford Heath <no.spam@please.net>  
wrote:

> ...snip...
>
> Thanks, I will do.
>
>> ...snip...

I just tested it in a sample circuit, that new model is 1/3 the speed of  
the other model!

Envision grounded base with current source(s) to emitter. collector has 1k  
load with about 5mA bias current.
Also substrate voltage at -8Vdc.

Roll off of old model approx 190MHz, new model 64.5MHz! significant  
differences there.



Plus has a bit more noise, if that's important.

On Fri, 04 Apr 2014 14:28:00 -0700, Jim Thompson  
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

> On Fri, 04 Apr 2014 14:03:58 +1100, Clifford Heath
> <no.spam@please.net> wrote:
>
> [snip]
>
> You apparently have multiple line-wrap issues in your .asc file.
>
> Try not putting everything in one line ;-)
> 		
>                                         ...Jim Thompson


You have no idea how long I waited to be able to finally say this, "I had  
no problems with it." ;)

On 05/04/14 11:49, RobertMacy wrote:
> On Fri, 04 Apr 2014 13:58:46 -0700, Clifford Heath <no.spam@please.net>
> wrote:
>> ...snip...
>> Thanks, I will do.
>>> ...snip...
> I just tested it in a sample circuit, that new model is 1/3 the speed of
> the other model!
> Envision grounded base with current source(s) to emitter. collector has
> 1k load with about 5mA bias current.
> Also substrate voltage at -8Vdc.
>
> Roll off of old model approx 190MHz, new model 64.5MHz! significant
> differences there.

Wow, yes, big difference. However the Ft of the CA3046 is meant to be 
around 300MHz, whereas the LM3046 is min 300, typical 550. I don't 
really know how much gain your test circuit was asking for, but they do 
say "DC to 120MHz" on the National data sheet... Maybe I'm asking too 
much to push to 150MHz, but surely not to 30?

That said, I was just playing with parallel R loading the tank, and it 
doesn't take much to drop the amplitude way down... it seems likely that 
my original problem was excessive tank loading.

> Plus has a bit more noise, if that's important.

Make it a bit more likely to start perhaps :P

--
Clifford Heath

On 05/04/14 00:25, Chris Jones wrote:
> On 04/04/2014 19:44, Clifford Heath wrote:
> [snip]
> > <http://cjh.polyplex.org/electronics/fox/Oscillator.brd.png>
 > > <http://cjh.polyplex.org/electronics/fox/Oscillator.sch.png>

> I note that you are taking the output via C9 from the top of the tank,
> which is a very sensitive node. I hope it is going to something with
> really low loss, or that could be a cause for reducing the Q and
> tendency to oscillate. I would suggest taking the output from some
> low-impedance node, such as a tap on the inductor, or the top of R6 or
> something like that.

I was taking signal off the tank because the signal level is 20dB better 
than at the emitter, while the spurious levels are the same.

I've switched the design to a tap off the inductor, which is still clean 
but much lower impedance (less tank load). I need to re-do the layout 
before retesting at a higher frequency, but all the symptoms I am seeing 
are explained by excess loading (or various sorts) on the tank.

I just got bitten yesterday by the very high Cbe of the 2N2222. 20pF 
indeed! I'll be more careful using those in future :)

Thanks everyone for your help.

On 15/04/2014 10:09, Clifford Heath wrote:
> On 05/04/14 00:25, Chris Jones wrote:
>> On 04/04/2014 19:44, Clifford Heath wrote:
>> [snip]
>> > <http://cjh.polyplex.org/electronics/fox/Oscillator.brd.png>
>  > > <http://cjh.polyplex.org/electronics/fox/Oscillator.sch.png>
>
>> I note that you are taking the output via C9 from the top of the tank,
>> which is a very sensitive node. I hope it is going to something with
>> really low loss, or that could be a cause for reducing the Q and
>> tendency to oscillate. I would suggest taking the output from some
>> low-impedance node, such as a tap on the inductor, or the top of R6 or
>> something like that.
>
> I was taking signal off the tank because the signal level is 20dB better
> than at the emitter, while the spurious levels are the same.
Yes, fair enough. If you have a load with very high impedance (the gate 
of a very tiny fet perhaps) then taking the output from the large swing 
of the tank can give you the best phase noise floor at high offsets, but 
that load needs to be very high impedance. With a low impedance load it 
can stop it oscillating.

>
> I've switched the design to a tap off the inductor, which is still clean
> but much lower impedance (less tank load). I need to re-do the layout
> before retesting at a higher frequency, but all the symptoms I am seeing
> are explained by excess loading (or various sorts) on the tank.
>
> I just got bitten yesterday by the very high Cbe of the 2N2222. 20pF
> indeed! I'll be more careful using those in future :)
>
> Thanks everyone for your help.